A Slice of Slurry Rheology
What is electrode slurry rheology and why should we care about it? We previously wrote about dry electrode manufacturing and the difficulties in manufacturing high-quality cells. Pooja drops the facts on slurry rheology.
Today we want to take this idea further and understand what metrics are important when making electrode slurry to optimise the coating process and enable a high-performance battery. A quick disclaimer, there will be a lot of cake terminology to help explain some of the processes so please do not blame this article for any cake consumption after reading (check out Jill Pestana’s dessert cake battery recipe).
Takeaways
Understanding the main rheology properties of electrode slurries:
The viscosity of the electrode binder solution - affects the ease at which particles are dispersed and mixed.
Response of non-Newtonian fluids - influences coating performance and speed at which a uniform coating can be applied.
Yield stress and stability of the mixture - affects slurry stability and whether particles are uniformly suspended in the mixture.
Slurry Making
The slurry of the anode or cathode is a mixture of solid particles usually suspended in a solvent (for the wet electrode coating process). This complex mixture of solids and liquids consists of active material (anode/cathode), binders, electronically conductive particles and a solvent medium. The slurry is coated onto the current collector substrate using a doctor-blade or slot die coating technique which is dried and calendared to make a dense electrode layer. The electrodes are then ready to be cut, made into cells and formed and conditioned on a battery cycler for electrochemical testing, a schematic of this process can be seen below:
The manufacturing process of these slurry suspensions is important as it describes their deformation and flow behaviour which will dictate the final electrode thickness, porosity, coverage of film gaps and rate of drainage. We want to synthesise homogenous, defect-free electrodes that have active material evenly dispersed within them to achieve the target weights. Common electrode slurry ingredients are listed:
Characterising the rheology of electrode slurries throughout the manufacturing stages is imperative. One of the most important rheology properties is the slurry viscosity as it controls how well the slurry ingredients are mixed and the film coverage. Before we dive into viscosity, shear-thinning, non-Newtonian fluids, yield stress and viscoelasticity, let’s try to break this down and visualise the slurry coating process. What better way to explain slurry rheology than a baking analogy?
Rheology 101
Let’s start with the basics first. Rheology is the area of physics that describes the deformation and flow of matter in particular non-Newtonian (we will explain this later) liquids and the plastic flow of solids. Viscosity is a property of rheology and is the measure of the shear stress divided by the shear rate. If we imagine spreading icing on top of a cake with a knife, the shear rate is the speed of the knife divided by the distance between the bottom of the knife and the top of the cake with the icing sandwiched between the two:
Shear rate (1/s) = speed of knife (or coating speed m/s) / distance between the bottom of the knife and top of the cake (or coating gap, m)
If we take water, an important ingredient used in baking to wet any flour mixture, it can be described as a Newtonian fluid. This means if we were to draw a plot of shear stress versus shear rate it would follow a straight line with a constant slope at a given temperature (blue line in the figure below). However, not all fluids obey the Newtonian relationship, and the fluid’s viscosity alters in response to the shear rate. A plot of stress versus strain for different types of non-Newtonian fluids can be seen in the figure below, we will discuss each in detail.
Viscosity
There are 5 main types of non-Newtonian fluids:
Cake batter is an example of a pseudoplastic fluid. If you imagine spreading cake batter along a flat surface (e.g. a baking tin), the quicker the batter is spread (higher shear rate), the lower the cake batter viscosity becomes. This type of response is also called shear thinning – the viscosity reduces with an increased shear rate.
Corn starch mixed with water is an example of a dilatant fluid. For example, if we try to spread the corn starch mixture faster (higher shear rate), the mixture starts to thicken further i.e. the viscosity increases with shear rate which is also known as shear thickening.
Ketchup is an example of a Bingham fluid meaning that a non-zero shear stress is required before it will flow at all. Have you tried to squeeze ketchup out of a bottle and realised a certain amount of pressure is required before it squirts everywhere?
Yoghurt and honey are great examples of thixotropic fluid. When a constant shear rate is applied to honey for example spreading honey on toast at a constant rate, the viscosity of honey decreases.
As cream turns to whipped cream it displays an excellent example of a rheopexy fluid i.e. when a constant shear rate is applied the viscosity of the cream thickens over time to form whipped cream.
Yield Stress
Now that we have understood viscosity, let’s understand another important material property: yield stress. For any high solid content suspensions or slurries (e.g. cake batter or electrode slurries) the yield stress plays a crucial role. If the applied shear stress is lower than the yield stress the slurry will not deform and therefore will behave like a solid and not flow like a liquid. In electrode slurry casting, usually the shear stress or rate is sufficiently higher than the yield stress of the slurry and the subsequent drying stage where the solvent is evaporated, results in a higher solid content which increases the yield stress. This also happens when we bake a cake (water has evaporated) and as we take it out of the oven to let it “stand” or cool to room temperature.
Anode and Cathode Rheology
We have explained the main rheology considerations in terms of baking, let’s try to bring the focus back to battery electrodes and understand the importance of slurry composition. An illustration of the electrode slurry for the anode (left) and cathode (right) can be seen:
Cake Battery Analogy
The anode consists of the active material (graphite), mixed with carbon black (CB) conductive agent and carboxymethyl cellulose (CMC) and styrene-butadiene rubber (SBR) binders which are dispersed in an aqueous solution.
The cathode consists of the active material (NMC), mixed with CB and polyvinylidene fluoride (PVDF) binder in a N-Methyl-2-pyrrolidone (NMP) solvent.
The binders play an important role in both of these electrodes. For example, for aqueous-based anodes they are critical for the adsorption of the binder system: CMC and SBR onto the graphite have shown to be important for homogenous dispersion by producing electrostatic repulsion between particles. The binder concentration is another parameter that needs to be carefully controlled, at high CMC concentrations, the structure is a complex tangled polymer matrix. However, at lower concentrations where particle-particle interactions become important, more CMC can adsorb to graphite which decreases the slurry viscosity and increases current collector adhesion when the electrode is dried.
Let’s think about baking again. Imagine a cake batter made up of diluted milk (behaves like water), egg and flour. The main purpose of the egg is to act as a binder (like the CMC) and keep the final products intact. With a high concentration of egg to flour ratio, the egg that can add smoothness up to a point will actually act as a drying agent making the cake dense and rubbery. However, when less egg is added to the batter relative to the flour and diluted milk, the egg finds it easier to mix together with the cake batter making the texture smoother adding structural strength as batter viscosity decreases. If too little egg is added then the result is a very dense and brittle cake which falls apart easily.
Let’s consider the cathode material. The dispersion of CB forms a conductive network more easily in NMP than in a water solvent. The coating of cathode active material in a PVDF polymeric binder could be the reason for better adhesion and flexibility than in water-based anodes. One of the main roles of SBR in the water-based anode is to help the flexibility and adhesion properties. When considering different binder materials it is important to note a binder must not thicken the slurry whilst adsorbing to the surface of the active material, thereby displacing the CB. If the binder is not able to achieve this, then more CB would have to be used which would reduce the CB to active material ratio, thereby reducing the energy density. For cathodes such as LFP which have a much lower electronic conductivity than NMC, it is essential that the binder adsorbs onto the active material allowing free CB to form a conductive network in the solution to give it the desired rheology and conductive electrode properties.
I think you can guess what we are going to do next – that’s right we are going to take another baking analogy to help explain this! Let’s think about the cake batter with flour, egg and butter in full-fat milk this time (NMP solvent). The cake flour is the active material (cathode) and it is mixed with the butter (CB) and milk. When the egg (PVDF) is added to the mixture, if the egg is too cold it can cause the butter to seize up and the emulsion is broken i.e. some of the butter is dispersed from the flour, milk and egg mixture. Now, for our cathode material, the egg or PVDF isn’t actually too cold but has a tendency to preferentially adsorb to the flour (active material). However, the adsorption properties of the binder (egg) can be tuned which impacts how much CB (butter) is dispersed.
Summary
Both electrodes exhibit shear thinning behaviour but their rheology differs slightly due to the different materials used. In water-based anodes, the CMC (remember the role of egg in the cake?) dictates the rheology as it helps to disperse the graphite, CB and SBR. Therefore to optimise for anode rheology properties, the CMC binder should be tuned by altering concentration, molecular weight and degree of substitution.
For NMP cathodes, the CB (remember the butter?) dictates the final rheology as the CB adsorbs to the active material removing it from the NMP solution, altering its viscosity. When the PVDF binder is present, it can preferentially adsorb to the active material which frees some of the CB back into the solution. Therefore to tune the cathode rheology, the focus should be the amount of free CB which is the amount of available CB (amount of CB added) and the free active material surface the CB can adsorb to. A delicate balance has to be struck so that there is enough CB in the solution to form a network (suspension of NMP and CB for electronic conductivity properties) and not having too much CB that the rheology becomes gel-like (or too buttery in our cake mixture) resulting in uneven electrode coating.
We hope this has shed light on a few key rheology metrics and definitions. We have mainly focussed on slurry rheology and the role of binders, solvents and additives that are useful for optimising electrode slurries. Other properties such as slurry ink concentration, active material particle size distribution, the ratio of solid content, electrode drying rate, temperature and time, coating speed and additional additives or surfactants will also affect the final dried electrode rheology. An understanding of rheology is required to optimise the electrode formulation for high-performance batteries! We hope this article has been insightful…and maybe you’ve also learnt how to make a perfect cake!
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